Star Clusters

Sometimes stars just like to hang out together and they end up in "clusters". OK, that isn't a very scientific description but it will have to do. Clusters come in one of two types
and are held together by gravitational attraction. They are either open cllusters or globular clusters. Open clusters contain anywhere from a dozen to several hundred stars
usually in an unsymmetrical arrangement. Globular clusters by contrast are older systems containing thousands or even hundreds of thousands stars closely packed in a symmetrical, roughly
spherical form.

Examples of both types are shown below.

The Pleiades

During the cold winter months stargazers in the northern hemisphere are treated to a beautiful open star cluster called the Pleiades or Seven Sisters. Its official designations are M45 and
Melotte 22. Hot blue stars formed within the past 100 million years dominate this cluster. Astronomers estimate that in about 250 million years tidal forces will tear it apart and it will have
moved from its present location in Taurus to neighboring Orion.

The Great Hercules Cluster (M13) is considered by many to be the finest globular cluster in the northern half of the heavens. It contains an estimated 300,000 stars whose age is estimated to be
between 11 - 12 billion years. The stars are incredibly dense (by star standards) in the core region. As an example, if our planet was located in the core area we would see over 1000 stars at
night that were brighter than the brightest star we currently see - Sirius.

Globular cluster M15 is located in Pegasus and is perhaps the most dense cluster in our galaxy having undergone a process of contraction. This ball of stars measures approximately 210 light years
across, yet more than half of the stars you see are packed into the central area in a space just slightly more than 10 light years in size.